On Thu, 16 Jul 2026, Tamar Christina wrote:
> In GCC 16 the commit r16-7193-g158ad5f96954da5fa24d5c2a91ae92417fb62e20
> caused a big regression in performance of regex in libstdc++.
>
> This and other patches were tested using the CPP version of the benchmarks
> at https://github.com/mariomka/regex-benchmark/
>
> The benchmark uses regex_token_iterator over the input text with these
> patterns:
>
> - email: [\w.+-]+@[\w.-]+\.[\w.-]+
> - URI: [\w]+:\/\/[^\/\s?#]+[^\s?#]+(?:\?[^\s#]*)?(?:#[^\s]*)?
> - IPv4:
> (?:(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9])\.){3}(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9])
>
> Where it tests various types of regexpr. "email" has no backtracking so it's
> the
> simplest NFA possible which we should be able to handle quickly.
>
> "uri" has optional matches and some non capturing groups and "ipv4" adds some
> alternative matching to the equation.
>
> The input string is a 6.52 mb test file "input-text.txt"
>
> The regressions of each type of regexpr compared to GCC 15 are:
>
> email: 118.9%
> URI: 124.8%
> IPv4: 139.8%
>
> So most matches became > 2x slower.
>
> This patch series addresses the regressions and gets the new code to be
> ultimately faster than the GCC 15 implementation.
>
> Currently the _Executor class implements both a DFS and a BFS traveral mode
> for the regex matching. It has a parameter _M_search_mode which it uses
> inside the _Executor functions to separate out the implementations.
>
> However this parameter is rather opague to IPA and so for each implementation
> the other branch is always dead but they haven't been folded away.
>
> This increases the number of dynamic instructions and branches being executed
> and the branches seem to be often mispredicted.
Why are the branches often mispredicted if they never change?
>
> The patch fixes it by moving _Search_mode out of the class and making it a
> template parameter instead so we can at compile time fold away the two
> implementations. This removes all the extra compare and branches from the
> hot functions.
>
> These changes improve the benchmarks compared with GCC 16 with
>
> email: 17.8%
> URI: 16.8%
> IPv4: 8.9%
These measurements are with -O3 right? Shouldn't we also consider -O2?
I ask because this patch partially undoes r16-7425-gc49ce07bf09504 which
changed the compile-time search mode to a run-time flag for sake of code
size, run time, and compile time/memory usage according to my crude
benchmarks using -O2. I found that by eliminating the entire extra copy
of _Executor's implementation, its std::vector operations are more
likely to get inlined, which resulted in a 15% improvement in run time.
However, previously the entire _Executor template was parameterized on the
search mode and this patch does a more targeted approach of parameterizing
only the member functions that need it, which I hadn't considered doing.
This might be the best compromise, but I do wonder about performance
with -O2, since IIUC -O2 is less aggressive about inlining and we might
see again the performance degradation I saw when _Executor was fully
parameterized on the search mode, resulting in some hot std::vector
operations not getting inlined.
>
> On Neoverse-V1
>
> So there is still a regression until the end of the series and each patch
> will chip away at it.
>
> Also note that with none of these changes do I see an increase heap or stack
> usage that the original fix fixed. RSS stays about the same.
>
> PS. thanks for the link to the algorithm in the source, it was useful to
> understand how the machinery works!
>
> Bootstrapped Regtested on aarch64-none-linux-gnu,
> arm-none-linux-gnueabihf, x86_64-pc-linux-gnu
> -m32, -m64 and no issues.
>
> Ok for master?
>
> Thanks,
> Tamar
>
> libstdc++-v3/ChangeLog:
>
> PR libstdc++/126274
> * include/bits/regex_executor.h (_Search_mode): Move to top level.
> (_M_handle_repeat, _M_handle_match, _M_handle_accept, _M_node, _M_dfs):
> Add template parameter.
> (_M_visited): Add inline keyword.
> * include/bits/regex_executor.tcc (_M_handle_repeat, _M_handle_match,
> _M_handle_accept, _M_node, _M_dfs): Use template parameter.
>
> ---
> diff --git a/libstdc++-v3/include/bits/regex_executor.h
> b/libstdc++-v3/include/bits/regex_executor.h
> index
> 797ad702784b6d1bf07734d91683946d989630f5..f6e55f2f4aaa8fb3fa8d963b04d5bcf0b3b191d3
> 100644
> --- a/libstdc++-v3/include/bits/regex_executor.h
> +++ b/libstdc++-v3/include/bits/regex_executor.h
> @@ -50,10 +50,11 @@ namespace __detail
> * The %_Executor class has two modes: DFS mode and BFS mode, controlled
> * by the function parameter %__search_mode.
> */
> + enum class _Search_mode : unsigned char { _BFS = 0, _DFS = 1 };
> +
> template<typename _BiIter, typename _Alloc, typename _TraitsT>
> class _Executor
> {
> - enum class _Search_mode : unsigned char { _BFS = 0, _DFS = 1 };
> enum class _Match_mode : unsigned char { _Exact, _Prefix };
>
> public:
> @@ -116,7 +117,8 @@ namespace __detail
> void
> _M_rep_once_more(_Match_mode __match_mode, _StateIdT);
>
> - void
> + template<_Search_mode __search_mode>
> + void
> _M_handle_repeat(_Match_mode, _StateIdT);
>
> void
> @@ -137,22 +139,26 @@ namespace __detail
> void
> _M_handle_subexpr_lookahead(_Match_mode, _StateIdT);
>
> - void
> + template<_Search_mode __search_mode>
> + void
> _M_handle_match(_Match_mode, _StateIdT);
>
> void
> _M_handle_backref(_Match_mode, _StateIdT);
>
> - void
> + template<_Search_mode __search_mode>
> + void
> _M_handle_accept(_Match_mode, _StateIdT);
>
> void
> _M_handle_alternative(_Match_mode, _StateIdT);
>
> - void
> + template<_Search_mode __search_mode>
> + void
> _M_node(_Match_mode, _StateIdT);
>
> - void
> + template<_Search_mode __search_mode>
> + void
> _M_dfs(_Match_mode __match_mode, _StateIdT __start);
>
> bool
> @@ -247,7 +253,7 @@ namespace __detail
> return (_M_re._M_automaton->_M_options() & __m) == __m;
> }
>
> - bool
> + inline bool
> _M_visited(_StateIdT __i)
> {
> if (_M_visited_states)
> diff --git a/libstdc++-v3/include/bits/regex_executor.tcc
> b/libstdc++-v3/include/bits/regex_executor.tcc
> index
> 167a7a345300868ed5e0852e328569aec47e3d0d..86f6c6240853d673f47099a5fd15fe84fc99316e
> 100644
> --- a/libstdc++-v3/include/bits/regex_executor.tcc
> +++ b/libstdc++-v3/include/bits/regex_executor.tcc
> @@ -165,7 +165,7 @@ namespace __detail
> _M_has_sol = false;
> *_M_get_sol_pos() = _BiIter();
> _M_cur_results = _M_results;
> - _M_dfs(__match_mode, _M_start);
> + _M_dfs<_Search_mode::_DFS>(__match_mode, _M_start);
> return _M_has_sol;
> }
>
> @@ -208,7 +208,7 @@ namespace __detail
> for (auto& __task : __old_queue)
> {
> _M_cur_results = _ResultsVec(std::move(__task.second), __alloc);
> - _M_dfs(__match_mode, __task.first);
> + _M_dfs<_Search_mode::_BFS>(__match_mode, __task.first);
> }
> if (__match_mode == _Match_mode::_Prefix)
> __ret |= _M_has_sol;
> @@ -281,6 +281,7 @@ namespace __detail
> // mean the same thing, and we need to choose the correct order under
> // given greedy mode.
> template<typename _BiIter, typename _Alloc, typename _TraitsT>
> + template<_Search_mode __search_mode>
> void _Executor<_BiIter, _Alloc, _TraitsT>::
> _M_handle_repeat(_Match_mode, _StateIdT __i)
> {
> @@ -288,7 +289,7 @@ namespace __detail
> // Greedy.
> if (!__state._M_neg)
> {
> - if (_M_search_mode == _Search_mode::_DFS)
> + if constexpr (__search_mode == _Search_mode::_DFS)
> // If it's DFS executor and already accepted, we're done.
> _M_frames.emplace_back(_S_fopcode_fallback_next, __state._M_next,
> _M_current);
> @@ -298,7 +299,7 @@ namespace __detail
> }
> else // Non-greedy mode
> {
> - if (_M_search_mode == _Search_mode::_DFS)
> + if constexpr (__search_mode == _Search_mode::_DFS)
> {
> // vice-versa.
> _M_frames.emplace_back(_S_fopcode_fallback_rep_once_more, __i,
> @@ -390,13 +391,14 @@ namespace __detail
> }
>
> template<typename _BiIter, typename _Alloc, typename _TraitsT>
> + template<_Search_mode __search_mode>
> void _Executor<_BiIter, _Alloc, _TraitsT>::
> _M_handle_match(_Match_mode, _StateIdT __i)
> {
> const auto& __state = _M_nfa[__i];
> if (_M_current == _M_end)
> return;
> - if (_M_search_mode == _Search_mode::_DFS)
> + if constexpr (__search_mode == _Search_mode::_DFS)
> {
> if (__state._M_matches(*_M_current))
> {
> @@ -487,10 +489,11 @@ namespace __detail
> }
>
> template<typename _BiIter, typename _Alloc, typename _TraitsT>
> + template<_Search_mode __search_mode>
> void _Executor<_BiIter, _Alloc, _TraitsT>::
> _M_handle_accept(_Match_mode __match_mode, _StateIdT)
> {
> - if (_M_search_mode == _Search_mode::_DFS)
> + if constexpr (__search_mode == _Search_mode::_DFS)
> {
> __glibcxx_assert(!_M_has_sol);
> if (__match_mode == _Match_mode::_Exact)
> @@ -562,19 +565,23 @@ namespace __detail
> }
>
> template<typename _BiIter, typename _Alloc, typename _TraitsT>
> + template<_Search_mode __search_mode>
> #ifdef __OPTIMIZE__
> [[__gnu__::__always_inline__]]
> #endif
> inline void _Executor<_BiIter, _Alloc, _TraitsT>::
> _M_node(_Match_mode __match_mode, _StateIdT __i)
> {
> - if (_M_visited(__i))
> - return;
> + // DFS has no _M_visited implementation as such don't even have the
> branch
> + // or the check in the call graph.
> + if constexpr (__search_mode == _Search_mode::_BFS)
> + if (_M_visited(__i))
> + return;
>
> switch (_M_nfa[__i]._M_opcode())
> {
> case _S_opcode_repeat:
> - _M_handle_repeat(__match_mode, __i); break;
> + _M_handle_repeat<__search_mode>(__match_mode, __i); break;
> case _S_opcode_subexpr_begin:
> _M_handle_subexpr_begin(__match_mode, __i); break;
> case _S_opcode_subexpr_end:
> @@ -588,15 +595,15 @@ namespace __detail
> case _S_opcode_subexpr_lookahead:
> _M_handle_subexpr_lookahead(__match_mode, __i); break;
> case _S_opcode_match:
> - _M_handle_match(__match_mode, __i); break;
> + _M_handle_match<__search_mode>(__match_mode, __i); break;
> case _S_opcode_backref:
> - if (_M_search_mode == _Search_mode::_DFS)
> + if constexpr (__search_mode == _Search_mode::_DFS)
> _M_handle_backref(__match_mode, __i);
> else
> __builtin_unreachable();
> break;
> case _S_opcode_accept:
> - _M_handle_accept(__match_mode, __i); break;
> + _M_handle_accept<__search_mode>(__match_mode, __i); break;
> case _S_opcode_alternative:
> _M_handle_alternative(__match_mode, __i); break;
> default:
> @@ -605,10 +612,10 @@ namespace __detail
> }
>
> template<typename _BiIter, typename _Alloc, typename _TraitsT>
> + template<_Search_mode __search_mode>
> void _Executor<_BiIter, _Alloc, _TraitsT>::
> _M_dfs(_Match_mode __match_mode, _StateIdT __start)
> {
> - const bool __dfs_mode = (_M_search_mode == _Search_mode::_DFS);
> _M_frames.emplace_back(_S_fopcode_next, __start);
>
> while (!_M_frames.empty())
> @@ -621,17 +628,17 @@ namespace __detail
> case _S_fopcode_fallback_next:
> if (_M_has_sol)
> break;
> - if (__dfs_mode)
> + if constexpr (__search_mode == _Search_mode::_DFS)
> _M_current = __frame._M_pos;
> [[__fallthrough__]];
> case _S_fopcode_next:
> - _M_node(__match_mode, __frame._M_state_id);
> + _M_node<__search_mode>(__match_mode, __frame._M_state_id);
> break;
>
> case _S_fopcode_fallback_rep_once_more:
> if (_M_has_sol)
> break;
> - if (__dfs_mode)
> + if constexpr (__search_mode == _Search_mode::_DFS)
> _M_current = __frame._M_pos;
> [[__fallthrough__]];
> case _S_fopcode_rep_once_more:
> @@ -641,7 +648,7 @@ namespace __detail
> case _S_fopcode_posix_alternative:
> _M_frames.emplace_back(_S_fopcode_merge_sol, 0, _M_has_sol);
> _M_frames.emplace_back(_S_fopcode_next, __frame._M_state_id);
> - if (__dfs_mode)
> + if constexpr (__search_mode == _Search_mode::_DFS)
> _M_current = __frame._M_pos;
> _M_has_sol = false;
> break;
>
>
> --
>
